Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. The STO comprises a stack including a spin injection layer (SIL) with PMA character, an oscillating field generation layer (FGL) with in-plane anisotropy, and a spacer between the SIL and FGL.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers typically referred to as a reference layer and free layer are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and atop electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode in a MRAM device. In another aspect, a MTJ element in a read head sensor may be based on a giant magnetoresistance (GMR) effect that relates to a spin valve structure where a reference layer and free layer are separated by a metal spacer. In sensor structures, the MTJ is formed between two shields and there is a hard bias layer adjacent to the MTJ element to provide longitudinal biasing for stabilizing the free layer magnetization.
A high performance MRAM MTJ element is characterized by a high tunneling magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the change in resistance observed by changing the magnetic state of the free layer. For Spin-MRAM (STT-MRAM), high anisotropy and greater thermal stability is achieved with a high Hc and high energy barrier Eb=KuV/KBT where Ku is the magnetic anisotropy, V is the switching magnetic volume, KB is the Boltzmann constant, and T is the measurement temperature. Furthermore, a high TMR ratio and resistance uniformity Rp_(cov), and a low switching current are desirable.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and other spintronic devices. Theoretical expressions predict that perpendicular magnetic devices have the potential to achieve a switching current lower than that of in-plane magnetic devices with the same magnetic anisotropy field according to S. Magnin et al. in Nat. Mater. 5, 210 (2006).
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials which utilize shape anisotropy. Thus, magnetic devices with PMA are advantageous for achieving a low switching current and high thermal stability. For spin torque applications, a reference layer with high Hc and low stray field is required. Preferably, a synthetic antiferromagnetic (SAF) reference layer is employed with a coupling layer (spacer) formed between two ferromagnetic layers (RL1 and RL2) having PMA in opposite directions. Several PMA material systems for RL1 and RL2 have been reported and include various ordered (i.e. L10) alloys, unordered alloys, and laminates represented by (Pt/Fe)n, (Pd/Co)n, (Ni/Co)n, and the like, where n is the lamination number. Magnetization direction for RL1 and RL2 is anti-parallel due to the RKKY coupling through the spacer layer which is typically Ru or Cu. However, there is a big challenge to increase the RKKY (anti-ferromagnetic) coupling strength to enhance magnetic stability and thermal stability of the reference layer to be compatible with semiconductor processes that reach as high as 400° C. or higher. Note that a higher annealing temperature of >350° C. is also useful in achieving an enhanced TMR ratio.
None of existing technology is known to provide high Hc and thermal stability in a PMA layer that will withstand high temperature processing up to 400° C. or greater which is required in semiconductor fabrication methods. Therefore, a low cost multilayer with high PMA, high Hc, and improved thermal stability is needed to enable PMA materials to be more widely accepted in a variety of magnetic device applications.